1. Field of the Invention
The present invention relates to spectral diffusion communication and, more particularly, to a code division multiple access apparatus which is capable of high-speed synchronization.
2. Description of the Related Art
Code division multiple access (CDMA) is capable of accepting users as long as setting of code synchronization is possible because communication quality is gradually degradated, and an increase in the number of users can be expected, whereas other multiple communications systems, for example, FDMA (frequency division multiple access) and TDMA (time division multiple access), are incapable of accepting more than a set number of users. Further, CDMA has excellent interference-resistant characteristics, signal secrecy and fading-resistant characteristics, and has a wide range of uses.
A CDMA communication apparatus causes the transmission apparatus to multiply baseband data to be transmitted by a diffusion code and to further multiply a carrier and transmit the resulting signal from an antenna. In the receiving apparatus, a diffusion code having the same phase as that of the diffusion code during transmission is prepared, and baseband data is taken using in-line correlation detection.
Meanwhile, in this CDMA communication apparatus, timing at which a diffusion code is generated in the receiving apparatus, that is, synchronization acquisition, becomes a problem. Conventionally, as a synchronization acquisition method, a digital sliding correlator shown in FIG. 8, a digital matched filter shown in FIG. 9, or the like is used. The digital sliding correlator causes a diffusion code to cycle earlier than the received signal, and causes a determination circuit having a DLL (Delay Locked Loop) or the like to perform synchronization pull-in.
Since this digital sliding correlator has a synchronization mechanism using a loop, it is possible to maintain stable synchronization. However, there are drawbacks in that the operation is unstable due to the balance of the correlator, and cycling of a maximum of one cycle of a PN (Pseudorandom Noise) code is required and so synchronization acquisition takes time.
The digital matched filter, formed of a shift register, performs synchronization acquisition by detecting a correlation peak by performing correlation integration of a known diffusion code and a received signal. This digital matched filter is able to perform higher-speed synchronization than a sliding correlator. However, there is a possibility that the presence timing of the correlation peak may become uncertain. Further, when the number of bits for one cycle of a diffusion code increases (e.g., several thousand), the number of shift registers increases, posing economical problems. Furthermore, there are problems in that both the above-described sliding correlator and digital matched filter have a large electric power consumption during standby (other than during signal reception).